1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) device and more particularly, to an active-matrix addressing LCD device of the lateral electric field type, such as the In-Plane Switching (IPS) type.
2. Description of the Related Art
Generally, the LCD device has the features such as low profile, reduced weight, and low power consumption. In particular, the active-matrix addressing LCD device that drives the respective pixels arranged vertically and horizontally in a matrix array with the active elements has ever been recognized as a high image quality flat-panel display device. Especially, the active-matrix addressing LCD device using thin-film transistors (TFTs) as the active elements for switching the respective pixels has been extensively diffused.
Most of active-matrix addressing LCD devices, which utilizes the electrooptic effects of the TN (Twisted Nematic) type liquid crystal material sandwiched by two substrates, display images by the application of electric field approximately vertical to the main surfaces of the substrates across the liquid crystal material to thereby cause displacement of the liquid crystal molecules of the said material. These LCD devices are termed the “vertical electric field” type. On the other hand, some of the active-matrix addressing LCD devices display images by the application of electric field approximately parallel to the main surfaces of the substrates across the liquid crystal material to thereby cause displacement of the liquid crystal molecules of the said material in the planes approximately parallel to the said main surfaces. These LCD devices have been known also, which are termed the “lateral electric field” type. Various improvements have ever been made not only for the vertical electric field type LCD devices but also for the lateral electric field type ones. Some of the improvements made for the latter will be exemplified below.
For example, an electrode structure using comb-tooth electrodes mated with each other in the lateral electric field type LCD device is disclosed in the Patent Document 1 (i.e., U.S. Pat. No. 3,807,831) issued in 1974 (see claim 1, FIGS. 1 to 4 and FIG. 11).
An electrode structure using comb-tooth electrodes mated with each other for reducing the parasitic capacitance between the common electrode and the drain bus lines or between the common electrode and the gate bus lines in the active-matrix addressing LCD device that utilizes the electrooptic effect of the TN type liquid crystal material is disclosed in the Patent Document 2 (i.e., Japanese Examined Patent Publication No 63-21907) published in 1988 (see claim 1, FIG. 7 and FIGS. 9 to 13).
A technique that realizes the generation of lateral electric field in the active-matrix addressing LCD device using TFTs is disclosed in the Patent Document 3 (i.e., Japanese Unexamined Patent Publication No. 7-036058) published in 1995 (see claims 1 and 5, and FIGS. 1 to 23). With this technique, the common electrode(s) and the image signal electrodes or the common electrode(s) and the liquid crystal driving electrodes are respectively formed on different layers from each other in such a way that an insulating film intervenes between them, and at the same time, the common electrode(s) or the liquid crystal driving electrodes is/are formed to be ring-, cross-, T-, Π-, H-, or ladder-shaped.
Furthermore, a technique that reduces the display coloring phenomenon in the slant views is disclosed in the Patent Document 4 (i.e., Japanese Unexamined Patent Publication No. 10-307295) published in 1998 (see claim 5, and FIGS. 4 and 6), where the electrodes for generating lateral electric field are bent to form bends, thereby intentionally making the driving (rotating) direction of the liquid crystal molecules different from each other in the respective regions with the bends of the said electrodes in the state where electric field is applied.
FIGS. 1 and 2 show an example of the structure of the lateral electric field type LCD device disclosed in the Patent Document 4. FIG. 1 is a plan view of the active-matrix substrate of this LCD device and FIG. 2 is a cross-sectional view of this LCD device along the line II-II in FIG. 1. Both of these figures show the structure of one of the pixel regions thereof because all the pixel regions have the same structure.
On the active-matrix substrate of the related-art LCD device shown in FIGS. 1 and 2, gate bus lines 155 are formed to extend along the lateral (or horizontal) direction of FIG. 1, and drain buss lines 156 are formed to extend along the vertical (or longitudinal) direction of FIG. 1 in such a way as to be bent repeatedly, thereby forming the pixel regions in the respective areas defined by the gate and drain buss lines 155 and 156. These pixel regions are arranged along the lateral and vertical directions of FIG. 1 to form a matrix array. In addition, common bus lines 152 are formed in parallel to the gate bus lines 155. One of the common bus lines 152 is located near the upper end positions of the pixel regions that are arranged laterally in one row of the matrix, and another of the common bus lines 152 is located near the lower end positions of the same pixel regions. Therefore, two of the common bus lines 152 are assigned to each pixel region. Thin-Film Transistors (TFTs) 145 are formed near the respective intersections of the gate bus lines 155 and the drain bus lines 156. Each of the pixel regions comprises one of the TFTs 145.
The gate bus lines 155 and the common bus lines 152 are straight; however, the drain bus lines 156 are bent to be V-shaped in the respective pixel regions, as shown in FIG. 1. Each pixel region is also bent to be V-shaped.
A pixel electrode 171 and a common electrode 172, each of which serves as a liquid crystal driving electrode for generating liquid crystal driving electric field, are formed in each pixel region. The pixel electrode 171 and the common electrode 172 are made of transparent conductive material.
In each pixel region, as shown in FIG. 1, the pixel electrode 171 is connected electrically and mechanically to the source electrode 142 of the corresponding TFT 145 and is laterally ladder-shaped. The part of the pixel electrode 171 that constitutes each rung of the ladder (i.e., the rung part of the pixel electrode 171) is bent to be V-shaped along the drain bus lines 156. The common electrode 172 is connected electrically and mechanically to the two common bus lines 152 located at the upper and lower positions of the pixel region and is laterally ladder-shaped also. The part of the common electrode 172 that constitutes each rung of the ladder (i.e., the rung part of the common electrode 172) is bent to be V-shaped along the drain bus lines 156.
The pixel electrode 171 comprises the three V-shaped rung parts and the common electrode 172 comprises the four V-shaped rung parts. The pixel electrode 171 and the common electrode 172 are arranged in such a way that their rung parts are alternately aligned at equal intervals along the lateral direction of FIG. 1. The pixel region is divided into the first sub-region 101 placed at the upper position of FIG. 1 and the second sub-region 102 placed at the lower position thereof.
The rung parts of the pixel electrode 171 and those of the common electrode 172 are shifted to each other clockwise by a predetermined angle with respect to the vertical direction of FIG. 1 in the first sub-region 101. In the second sub-region 102, the rung parts of the pixel electrode 171 and those of the common electrode 172 are shifted to each other counterclockwise by the same angle with respect to the vertical direction of FIG. 1.
As explained above, the source electrode 142 of the TFT 145 is connected electrically and mechanically to the corresponding pixel electrode 171. However, the drain electrode 144 of the same TFT 145 is connected electrically and mechanically to the corresponding drain bus line 156 which is located on the left side of FIG. 1. The gate electrode (not shown) of the same TFT 145 is connected electrically and mechanically to the corresponding gate bus line 155 which is located at the bottom of FIG. 1. The drain electrode 144 and the source electrode 142 are arranged at each side of the island-shaped semiconductor film 143 of the same TFT 145 in such a way as to be overlapped with the same semiconductor film 143.
As shown in FIG. 2, the common electrodes 172 are formed directly on the surface of a transparent plate 111 and are covered with an interlayer insulating film 157 formed on the surface of the plate 111. Although not shown in FIG. 2, the gate bus lines 155, the common bus lines 152, and the gate electrodes of the TFTs 145 are also formed directly on the surface of the plate 111 and are covered with the interlayer insulating film 157. The pixel electrodes 171 and the drain bus lines 156 are formed on the interlayer insulating film 157. Therefore, the pixel electrodes 171 and the drain bus lines 156 are electrically insulated from the common electrodes 172, the gate bus lines 155, the common bus lines 152, and the gate electrodes by the interlayer insulating film 157.
The pixel electrode 171 and the common electrode 172 are overlapped with each other in the pixel region in such a way that the interlayer insulating film 157 intervene between these electrodes 171 and 172, thereby forming additional capacitors with their overlapped parts.
Although not shown in FIG. 2, the source electrodes 142, the drain electrodes 144, and the semiconductor films 143 of the TFTs 145 are also formed on the Interlayer insulating film 157. Therefore, the source electrodes 142, the drain electrodes 144, and the semiconductor films 143 are electrically insulated from the common electrodes 172, the gate bus lines 155, the common bus lines 152, and the gate electrodes by the interlayer insulating film 157.
A protective insulating film 159 is formed on the interlayer insulating film 157. The pixel electrodes 171 and the drain bus lines 156, and the source electrodes 142, the drain electrodes 144 and the semiconductor films 143 of the TFTs 145 are covered with the protective insulating film 159.
An alignment film 131 made of an organic polymer is formed on the protective insulating film 157. The surface of the alignment film 131 has been subject to a predetermined aligning treatment.
On the other hand, an opposite substrate, which is coupled with the above-described active-matrix substrate, comprises a transparent plate 112 on which a color filter (not shown) including three primary colors, red (R), green (G), and blue (B), is formed corresponding to the respective pixel regions. A black matrix (not shown) for blocking light is formed on the plate 112 in the area excluding the regions corresponding to the pixel regions. A planarization film (not shown) is formed to cover the color filter and the black matrix on the plate 112. Moreover, columnar spacers (not shown) are formed on the planarization film. An alignment film 132 made of an organic polymer is formed on the planarization film so as to cover the spacers. The surface of the alignment film 132 has been subject to a predetermined aligning treatment.
The active-matrix substrate and the opposite substrate with the above-described structures are superposed with each other at a predetermined gap, where the surfaces of the substrates on which the alignment films 131 and 132 are respectively placed are faced inward. A liquid crystal material 120 is confined in the gap between the active-matrix and opposite substrates, forming a liquid crystal layer 122. A pair of polarizer plates (not shown) is arranged on the outer surfaces of the active-matrix and opposite substrates, respectively.
As shown in FIG. 1, the surfaces of the alignment films 131 and 132 have been subject to the aligning treatment uniformly in such a way that the liquid crystal molecules 121 existing in the liquid crystal layer 122 are aligned in parallel along the longitudinal (or vertical) direction of FIG. 1 when no electric field is applied. In the example of FIG. 1, the liquid crystal material 120 used is of the positive type. However, the liquid crystal material 120 may be of the negative type. In this case, the surfaces of the alignment films 131 and 132 will be respectively subject to aligning treatments in such a way that the liquid crystal molecules 121 are aligned in parallel to the lateral (or horizontal) direction of FIG. 1.
The penetration axes of the above-described pair of polarizer plates are intersected at right angles with each other. The penetration axis of one of the polarizer plates is in accordance with the initial alignment direction (i.e., the alignment direction when no electric field is applied) of the liquid crystal molecules 121.
Next, an example of the fabrication process steps of the related-art LCD device shown in FIGS. 1 and 2 will be explained below.
Regarding the active-matrix substrate, first, chromium (Cr) film is formed on the surface of the transparent plate 111 made of glass and is patterned to have a predetermined shape, forming the gate bus lines 155 and the common electrodes 172 on the surface of the plate 111. Next, a silicon nitride (SiNx) film is formed on the surface of the plate 111 to cover the gate bus lines 155 and the common electrodes 172 thus formed, resulting in the interlayer insulating film 157. Subsequently, a semiconductor film, which is used for forming the active layers of the TFTs 145, is formed on the interlayer insulating film 157 and patterned, forming the island-shaped semiconductor films 143 at the respective positions that overlap with the corresponding gate bus lines 155 in such a way that the interlayer insulating film 157 intervenes between them. Moreover, a Cr film is formed on the interlayer insulating film 157 and patterned, forming the source electrodes 142 and the drain electrodes 144, the drain bus lines 156, and the pixel electrodes 171 on the interlayer insulating film 157. Following this, a SiNx film is formed on the interlayer insulating film 157 to cover the source and drain electrodes 142 and 144, the drain bus lines 156, and the pixel electrodes 171, resulting in the protective insulating film 159. In this way, the active-matrix substrate is fabricated.
Regarding the opposite substrate, first, the color filter and the black matrix are selectively formed on the surface of the transparent plate 112 made of glass. Thereafter, the planarization film is formed to cover the color filter and the black matrix. Moreover the columnar spacers are formed on the planarization film. In this way, the opposite substrate is fabricated.
Following this, the alignment films 131 and 132, which are made of polyimide, are respectively formed on the surface of the active-matrix substrate and the surface of the opposite substrate with the above-described structures. Then, the surfaces of the alignment films 131 and 132 are uniformly subjected to the predetermined aligning treatments, respectively. Subsequently, the active-matrix substrate and the opposite substrate are superposed on each other to form a gap of 4.0 μm between them and thereafter, a nematic liquid crystal material whose refractive index anisotropy is 0.075 is injected into the gap between these two substrates in a vacuum chamber (not shown). Finally, the polarizer plates are respectively adhered onto the outer surfaces (i.e., backs) of the substrates. As a result, a LCD panel is fabricated.
A driver LSI (Large-Scale Integrated Circuit) and a backlight unit are mounted on the LCD panel thus fabricated. Thus, the fabrication process steps of the related-art LCD device shown in FIGS. 1 and 2 are completed.
With the related-art LCD device shown in FIGS. 1 and 2, when a voltage is applied, liquid crystal driving electric field is generated along a direction slightly inclined clockwise with respect to the lateral direction of FIG. 1 in the first sub-region 101 and at the same time, liquid crystal driving electric field is generated along a direction slightly inclined counterclockwise with respect to the lateral direction of FIG. 1 in the second sub-region 102. Therefore, the liquid crystal molecules 121, which have been uniformly aligned along the vertical direction of FIG. 1 when no electric field is applied, are rotated counterclockwise in the first sub-region 101 and clockwise in the second sub-region 102 by the liquid crystal driving electric field applied. In this way, the rotation directions of the liquid crystal molecules 121 in the first and second sub-regions 101 and 102 are made different from each other and consequently, the display coloring phenomenon dependent on the viewing angle change can be suppressed.
However, the above-described related-art LCD device shown in FIGS. 1 and 2 has the following problem.
Specifically, the related-art LCD device of FIGS. 1 and 2 does not have any stabilization structure for stabilizing the liquid crystal molecules 121 on the boundary between the first and second subregions 101 and 102 in each pixel region and therefore, distortion of the liquid crystal domains is likely to occur. In particular, when the size of liquid crystal domains is reduced in accordance with the smaller pixels for realizing higher resolution, for example, the adjoining liquid crystal domains tend to be merged indefinitely in shape, or the liquid crystal domain which should be present at a specific Location tends to disappear. In this way, such the distortion of the liquid crystal domains as described here is more likely to occur in the case of the miniaturized liquid crystal domains. As a result, a problem of display roughness and/or display unevenness will take place.
In particular, this problem will be conspicuous if the display screen of the LCD device is pressed with a finger or the like. Once the liquid crystal domains are distorted due to the finger pressing or the like, an appropriate measure, such as stopping the operation of the LCD device (i.e., electric power shutdown) and leaving the device as it is for a while, needs to be taken for recovering the liquid crystal domain distortion (which is seen as a finger-pressed mark) thus generated.
To solve this problem, another example of the electrode structure of the lateral electric field type LCD device disclosed in the Patent Document 4 is effective. This electrode structure is shown in FIG. 3.
The structure of the related-art LCD device shown in FIG. 3 is the same as that of the related-art LCD device of FIGS. 1 and 2 except that the pixel electrode 171 and the common electrode 172, which are formed on the active-matrix substrate, comprise respectively whisker-shaped boundary stabilization electrodes 195 and 196 formed at the bends 173 and 174 of the electrodes 171 and 172 in such a way as to protrude from the bends 173 and 174 along the boundary between the first and second sub-regions 101 and 102. Therefore, the explanation for the same structural elements will be omitted here by attaching the same reference numerals as those used in the above explanation for the device structure of FIGS. 1 and 2 to the same elements shown in FIG. 3. In addition, the structure of the opposite substrate is the same as that shown in FIG. 2.
Since the device structure of FIG. 3 comprises the whisker-shaped boundary stabilization electrodes 195 and 196, the rotation directions of the liquid crystal molecules 121 in the first and second sub-regions 101 and 102 do not become opposite to their desired rotation directions in the vicinities of the V-shaped bends 173 and 174 (in other words, on the boundary of the first and second sub-regions 101 and 102), respectively. This means that the rotation directions of the liquid crystal molecules 121 in the respective sub-regions 101 and 102 are stable. As a result, images can be displayed uniformly and stably with the device structure of FIG. 3.
Another device structure comprising the boundary stabilization electrodes 195 and 196 of FIG. 3 is shown in FIGS. 4 and 5. FIG. 4 is a plan view of the active-matrix substrate of the related-art LCD device with this structure, and FIG. 5 is a cross-sectional view of the LCD device along the line V-V in FIG. 4. The device structure of FIGS. 4 and 5 utilize the technique disclosed in the Patent Document 5 (i.e., Japanese Unexamined Patent Publication No. 2002-323706) published in 2002 (see FIGS. 68 and 69 and paragraphs 0426 to 0430).
The device structure of FIGS. 4 and 5 is the same as that of FIGS. 1 and 2 except for the following points (a) and (b). Therefore, the explanation for the same structural elements will be omitted here by attaching the same reference numerals as those used in the above explanation for the device structure of FIGS. 1 and 2 to the same elements shown in FIGS. 4 and 5. In addition, the structure of the opposite substrate is the same as that shown in FIG. 2.
The different points (a) and (b) of the structure of FIGS. 4 and 5 are:
(a) the pixel and common electrodes 171 and 172 serving as the liquid crystal driving electrodes formed on the active-matrix substrate are comb-tooth-shaped and arranged in such a way as to be mated with each other, where the pixel and common electrodes 171 and 172 are formed by a transparent conductive material film located on an upper layer (i.e. on a layer closer to the liquid crystal layer 120) than the gate bus lines 155 and the drain bus lines 156; and
(b) boundary stabilization electrodes 195a and 196a, which are in the electrically floating state, are formed in such a way as to be overlapped with the bends 173 and 174 of the pixel and common electrodes 171 and 172, respectively, where the boundary stabilization electrodes 195a and 196a are formed by a metal film located on the same layer as that of the gate bus lines 155 and the drain bus lines 156.
The related-art device structure of FIGS. 4 and 5 will be explained in detail below.
With the structure of FIGS. 4 and 5, similar to the related-art device structure of FIGS. 1 and 2, the interlayer insulating film 157 and the protective insulating film 159 are stacked on the surface of the transparent plate 111 in this order. In addition to the insulating films 157 and 159, an interlayer insulating film 160 is formed on the protective insulating film 159. Unlike the structure of FIGS. 1 and 2, both of the pixel electrodes 171 and the common electrodes 172 are formed on the interlayer insulating film 160.
The pixel electrode 171 comprises three comb-tooth parts extending like a V character along the V-shaped drain bus lines 156. The common electrode 172 comprises four comb-tooth parts extending like a V character along the V-shaped drain bus lines 156. The comb-tooth parts of the pixel and common electrodes 171 and 172 are alternately arranged at predetermined intervals along the gate bus lines 155 and mated with each other in the pixel region.
On the surface of the glass plate 111, auxiliary common electrodes 172a are formed in addition to the gate bus lines 155, the common bus lines 152, and the gate electrodes of the TFTs 145, which are covered with the interlayer insulating film 157. Two of the auxiliary common electrodes 172a are assigned to each pixel region. These auxiliary common electrodes 172a are respectively arranged in the vicinities of the two drain bus lines 156 located at each side of the pixel region and are extended along the same drain bus lines 156. The auxiliary common electrodes 172a are provided for electrical interconnection between the common electrodes 172 formed on the interlayer insulating film 160 and the corresponding common bus lines 152 formed on the transparent plate 111.
On the interlayer insulating film 157, auxiliary pixel electrodes 171a are formed in addition to the drain bus lines 156, and the source electrodes 142, the drain electrodes 144 and the semiconductor films 143 of the TFTs 145, which are covered with the protective insulating film 159. One of the auxiliary pixel electrodes 171a is assigned to each pixel region. The auxiliary pixel electrode 171a is overlapped with the central comb-tooth part of the pixel electrode 171 located at the center of the pixel region and is extended along the same. The auxiliary pixel electrode 171a is provided for electrical interconnection between the pixel electrode 171 formed on the interlayer insulating film 160 and the corresponding source electrode 142 formed on the interlayer insulating film 157. Therefore, the auxiliary pixel electrode 171a is connected electrically and mechanically to the corresponding source electrode 142 in the pixel region.
In each pixel region, the pixel electrode 171 is electrically connected to the corresponding source electrode 142 of the TFT 145 by way of a corresponding contact hole 161 that penetrates through the protective insulating film 159 and the interlayer insulating film 160, and the corresponding auxiliary pixel electrode 171a. The common electrode 172 is electrically connected to the corresponding common bus line 152 by way of a corresponding contact hole 162 that penetrates through the interlayer insulating film 157, the protective insulating film 159, and the interlayer insulating film 160, and the auxiliary common electrode 172a. 
The drain electrode 144 is connected electrically and mechanically to the corresponding drain bus line 156, and the gate electrode of the TFT 145 is connected electrically and mechanically to the corresponding gate bus line 155. These points are the same as the device structure of FIGS. 1 and 2. The alignment film 131 made of an organic polymer is formed on the interlayer insulating film 160, and the surface of the alignment film 131 has been subjected to a predetermined aligning treatment. These points also are the same as the device structure of FIGS. 1 and 2.
The structure of the opposite substrate is the same as that of the device structure of FIGS. 1 and 2. Thus, the explanation for the opposite substrate is omitted here.
Even with the related-art lateral electric field type LCD device shown in FIGS. 4 and 5, the electrically-floating boundary stabilization electrodes 195a and 196a are respectively provided for the pixel and common electrodes 171 and 172, and thus, the rotation directions of the liquid crystal molecules 121 near the border of the first and second sub-regions 101 and 102 are stabilized. As a result, images can be displayed uniformly and stably.
As seen from the above explanations, with the related-art device structure of FIG. 3 where the boundary stabilization electrodes 195 and 196 are respectively provided at the bends 173 and 174 of the pixel and common electrodes 171 and 172, and with the related-art device structure of FIGS. 4 and 5 where the electrically-floating boundary stabilization electrodes 195a and 196a are respectively provided for the pixel and common electrodes 171 and 172, the rotation directions of the liquid crystal molecules 121 near the bends 173 and 174 (in other words, near the boundary between the first and second sub-regions 101 and 102) are controlled more accurately, compared with the related-art device structure of FIGS. 1 and 2 that comprises no boundary stabilization electrodes. For this reason, the previously-described phenomenon that a finger-pressed mark remains for a long time on the display screen due to abnormal liquid crystal domains is difficult to occur.
However, even if one of these related-art device structures is used, the minute curvature of disclination lines (which is caused by abnormal alignment of the liquid crystal molecules 121) at the bends 173 and 174 cannot be controlled precisely. For this reason, a problem that a slight finger-pressed mark is inevitably perceived by the user if he/she looks at the display screen of the LCD device obliquely arises. According to the inventor's search, it was found that this problem is caused by the following reason:
Specifically, as shown in FIG. 6, when a part A of the display screen on the boundary between the first and second sub-regions 101 and 102, which is defined by a circle illustrated by a dot-dash line, is pressed with a finger, disclination (i.e., abnormal alignment) will occur near the pressed part A, even if the boundary stabilization electrodes 195 and 196 are formed at the bends 173 and 174. Moreover, disclination lines 1101 are formed in such a way as to curve and deviate from the line of symmetry of the bends 173 and 174 toward its right or left side (in other words, upward or downward in FIG. 6). The right-curved disclination line 1101 and the left-curved disclination line 1101 are equally likely to occur. Accordingly, there is a possibility that the curvature or state of the disclination lines 1101 in the finger-pressed area A is changed from the pattern of FIG. 6 to the pattern of FIG. 7.
Originally, in the places positioned at each side of the border of the first and second sub-regions 101 and 102, the rotation of the liquid crystal molecules 121 caused by the action of the applied liquid crystal driving electric field are different in direction from each other; therefore, the effects of the liquid crystal driving electric field to the obliquely incoming incident light are different from each other in these places. Here, the fact that the curvature of the disclination line 1101 on the border of the first and second sub-regions 101 and 102 is changed by pressing the display screen with a finger has a meaning that the size (or area) balance of these places where the rotation directions of the liquid crystal molecules 121 are different will deviate partially, although the quantity of this deviation is slight. Accordingly, when the user looks at the display screen obliquely, such the slight deviation of the size (or area) balance is observed as a slight finger pressed mark.